Petroleum refining and petrochemical processes frequently involve separating hydrocarbon components that have very similar structure and properties.
For example, propylene-propane splitters typically comprise distillation towers that are used to separate C3 streams into polymer grade propylene (PGP) in a net overhead and propane in a net bottoms. Due to the low relative volatility of propylene and propane, typically a very large tower with 150 to 250 trays is used. Additionally, the tower also typically includes a reflux to feed ratio of 5 to 10. Since the relative volatility of propylene and propane is so low (typically 1.05 to 1.20), the energy required to separate propylene and propane into high purity component streams is very high.
Typically, a heat pump compressor is utilized to condense (or remove energy) in the fractionation column overhead and re-boil (or feed energy) into the column bottoms because the vapor pressure of propylene and propane are similar and the heat removed from the column overhead for condensing can be transferred or pumped to the tower bottoms for re-boiling.
In a traditional design, such as the design disclosed in U.S. Pat. Pub. No. 2013/0131417, which is assigned to the assignee of the present invention, and the entirety of which is incorporated herein by reference, an overhead vapor from a propylene-propane splitter column (“PP Splitter”) is sent to the first stage heat pump compressor. In the first stage heat pump compressor, the vapor is compressed to the required pressure, typically between approximately 1034 to 1724 kPag (150 to 250 psig), which is the minimum temperature for a heat exchanger to condense vapor on the hot side and re-boil liquid on the cold side of the heat exchanger. The duty required to re-boil the PP Splitter determines the vapor flow rate to the re-boiler/condensers. Since the condensing duty is greater than the re-boiling duty of PP Splitter, there is extra vapor from the first stage discharge that requires heat removal. This extra vapor is sent to the second stage of the heat pump compressor, where it can be compressed to a pressure able to be condensed by another heat exchanger at a warmer temperature. Subsequently, this stream is flashed across a valve into a suction drum down to the column overhead pressure to provide Joule-Thomson effect cooling to the column overhead and accumulate propylene liquid product in the suction drum.
In such a system described above, when the second stage discharge stream is flashed down to the column overhead pressure, the resulting vapor from this flash is then re-processed in the heat pump first stage and second stages, sequentially. Thus, the first stage of the heat pump compressor, which is the larger capacity stage requiring more utility, needs to process the column overhead vapor along with the vapor from the second stage discharge flash, thereby increasing the overall capacity and utility requirement of the compressor.
It would be desirable to have a process which improves on the efficiency and heat recovery on such a process.